The present invention relates to applications in which a pivoting or a translating action occurs between two members. More particularly, it relates to a pivot for translation with high stiffness locking and infinite positioning in both directions while maintaining a relatively low release force.
In some applications, bi-directional locking is useful between two members moving relative to each other. Such applications include seat back adjustments for electric wheel chairs, leg support adjustments for wheel chairs, adjustable handles for power-assist manipulators, TV or monitor supports, and other relative movement applications. In such applications, accurate and infinite positioning under reversing load conditions is useful. Such applications also involve significant loading forces.
In these applications, typically bi-directional locking is desired and also high stiffness, which gives the user a tactile feeling of solidity. Furthermore, it is attempted to minimize deflection under load, that is, the feeling of “springiness”. Often these devices will not release while under load, and will only release when the load drops below a particular threshold level, and an input is deliberately applied to break the release mechanism. In some applications, bi-directional dual wrap spring devices are used for this function. Under reversing load, however, the output of a dual wrap spring device will begin reverse movement through a zone of reduced torque and then increasing torque, until the break locks in the reverse direction. This movement between full breaking engagement when reversing directions is sometimes referred to as “backlash” or “low system stiffness.” It is often difficult to minimize or prevent this movement under reversing load.
Other applications use opposed straight roller ramp locks in an attempt to overcome low system stiffness. Straight rollers on a straight shaft, however, have a tendency to “self lock” requiring large release forces after the applied load is removed. For these and other reasons, a need exists for the present invention.